The present invention relates to a liquid ejecting method and a liquid ejecting head which are used for ejecting droplets of liquid such as ink toward various recording media, such as paper, for the purpose of recording. In particular, it relates to a liquid ejecting method for ejecting extremely small droplets of liquid at an extremely high frequency, and also, a liquid ejecting head, that is, a recording head, which comprises a plurality of liquid paths arranged at a high density to realize high resolution.
Among various liquid ejecting methods are so-called bubble jet type liquid ejecting methods. According to these methods, bubbles are rapidly grown in liquid, and the pressure generated by the bubble grown is used to eject droplets of liquid from liquid ejection orifices. These methods are high in liquid ejection response, and therefore, are excellent for high speed recording and high density recording.
Among the bubble jet type liquid ejecting methods are liquid ejection methods which allow a bubble generated on a heat generating member to open to the atmosphere at the edge of an ejection orifice. As for such methods, Japanese Laid-Open Patent Application No 10940/1992, 10941/1992, 10742/1992, and the like, are well known.
These methods have following characteristics. First, they can increase liquid ejection velocity, and therefore, can increase reliability. Secondly, they can eject substantially the entire liquid present between a heat generating member and an ejection orifice, and therefore, can unify the volume by which liquid is ejected each time, which in turn reduces irregularity in terms of the image density.
As recording technologies progress, it has come to be required to record extremely high quality images, that is, to deposit liquid droplets of an extremely small volume (for example, 1.5xc3x9710xe2x88x9210 m3 or less) on recording medium at an extremely high density (for example, 600 dots/25.4 mm or more). In order to record such highly precise images, ejection orifices, and liquid paths leading to the ejection orifices, must be arranged at an extremely high density. For example, in order to accomplish the aforementioned recording density of 600 dots/25.4 mm, the ejection orifices must be aligned in two parallel lines, at a density of 300 unit/25.4 mm, the units in one line being displaced by half a pitch from the units in the other line in the line direction.
Recording an image with the use of finer liquid droplets increases the number of liquid droplets to be ejected, which in turn reduces recording speed. In order to prevent this recording speed reduction, it is necessary to increase the frequency at which liquid droplets are ejected from each ejection orifice per unit of time (hereinafter, xe2x80x9cejection frequencyxe2x80x9d). For example, in the case of the structure described above, the ejection frequency must be at least 7 kHz.
Further, in order to record a high quality image by ejecting liquid droplets with a volume as small as the one described above, the reliability with which liquid droplets are ejected must be improved.
As described above, there are bubble jet type liquid ejecting method which allow bubbles to become connected to the atmosphere. For example, Japanese Lair-Open Patent Application No. 16365/1993 discloses a technology regarding the state of a liquid droplet at the time of ejection, and the condition for allowing a bubble to become connected to the atmosphere.
When a bubble jet type liquid ejecting method which allows a bubble to become connected to the atmosphere was applied to an ink jet head which ejected extremely small liquid droplets with a volume of 1.5xc3x9710xe2x88x9210 m3, it was confirmed that during a recording operation, liquid droplets suddenly failed to be ejected from some of the ejection orifices through which liquid droplets had been properly ejected. This phenomenon was different from the ejection failure which occurred to the prior liquid ejecting heads. The investigation of this phenomenon revealed the following. That is, recording liquid suddenly plugged the ejection orifices during the period between the time when a bubble became connected to the atmosphere and the time when the refilling ended. Thereafter, recording liquid could not be ejected from the plugged ejection orifices unless a recovery operation was carried out with the use of the recovery mechanism of the main assembly of an image forming apparatus.
FIG. 5 is a section of a liquid ejection orifice, and a liquid path leading to the orifice, which depicts the above described phenomenon. As is evident from FIG. 5, immediately after a bubble becomes connected to the atmosphere and a droplet of recording liquid 501 is ejected, an ejection orifice is plugged with recording liquid 501. At this point of time, there also remains recording liquid 501 in the ink supply path. However, there is no recording liquid adjacent to an electrothermal transducer 1, because it is immediately after liquid ejection. In other words, there is only atmospheric air 502 adjacent to the electrothermal transducer 1. In this state, even if an electrical pulse is applied to the electrothermal transducer 1, a droplet of recording liquid 501 cannot be ejected, since there is no recording liquid 501 around the electrothermal transducer 1. Therefore, it is impossible to unplug the ejection orifice 4.
Further, during the development of the present invention, it became evident that when the aforementioned type of head, in which a large number of liquid paths were disposed at a high density, was driven at a high frequency, attention must be paid to the state of the meniscus after a bubble became connected to the atmosphere, in particular, how the state of the meniscus after the connection is different from the state of the meniscus prior to the connection. Thus, the object of the present invention is to provide a reliable liquid ejection method, that is, a liquid ejecting method which does not suddenly fail to eject liquid, i.e., a liquid ejecting method which makes high speed recording possible with the use of a bubble jet type liquid ejecting head, in particular, so-called side shooter type liquid ejecting head in which ejection orifices for ejecting extremely small liquid droplets at a high frequency are disposed at a high density, directly facing heat generating members one for one, and in which a bubble is allowed to become connected to the atmosphere.
The gist of the present invention for accomplishing the above-described object of the present invention is as follows.
The liquid ejecting method in accordance with the present invention uses a liquid ejecting head which comprises a plurality of electrothermal transducers capable of generating a sufficient amount of thermal energy for generating bubbles in liquid, a plurality of ejection orifices disposed directly facing the electrothermal transducers one for one, and a plurality of liquid paths. The ejection orifices are aligned at a density of no less than 300 per 25.4 mm, and are connected to the liquid paths one for one. This liquid ejecting method is characterized in that bubbles generated by the thermal energy generated by an electrothermal transducer eject droplets of liquid with a volume of no more than 15xc3x9710xe2x88x9215 m3, one for one, at a frequency of no less than 7 kHz, and open to the atmosphere as they eject the liquid while their internal pressure is below the atmospheric pressure, and that the height of the liquid path in the liquid ejecting head is no less than 6 xcexcm, and the distance between the top and bottom openings of the ejection orifice is no more than half the minimum distance across the ejection orifice through the center of the orifice.
The liquid ejecting head in accordance with the present invention comprises a plurality of electrothermal transducers capable of generating thermal energy for generating bubbles in liquid, a plurality of ejection orifices disposed directly facing the electrothermal transducers one for one, and a plurality of liquid paths. The ejection orifices and liquid paths are aligned at a density of no less than 300 per 25.4 mm. To the electrothermal transducers, driving signals are applied at a frequency of no less than 7 kHz. This liquid ejecting head is characterized in that bubbles are generated in the liquid paths, and eject droplets of liquid with a volume of no more than 15xc3x9710xe2x88x9215 m3, one for one, opening to the atmosphere as they eject the liquid while their internal pressure is below the atmospheric pressure, and that the height of the liquid path is no less than 6 xcexcm, and the distance between the top and bottom openings of the ejection orifice is no more than half the minimum distance across the ejection orifice through the center of the orifice.
Further, the liquid ejecting method in accordance with the present invention uses a liquid ejecting head which comprises a plurality of electrothermal transducers capable of generating a sufficient amount of thermal energy for generating bubbles in liquid, a plurality of ejection orifices disposed directly facing the electrothermal transducers one for one, and a plurality of liquid paths. The ejection orifices are aligned at a density of no less than 300 per 25.4 mm, and are connected to the liquid paths one for one. The bubbles generated by the thermal energy generated by an electrothermal transducer eject droplets of liquid with a volume of no more than 15xc3x9710xe2x88x9215 m3, one for one, at a frequency of no less than 7 kHz, and open to the atmosphere as they eject the liquid while their internal pressure is below the atmospheric pressure. This liquid ejecting method is characterized in that it comprises a process in which the liquid which remains within the ejection orifice after the bubble opens to the atmosphere, remains in connection to the liquid in the liquid path which retracts away from the ejection orifice, and a process in which the liquid remaining in the ejection orifice joins with the liquid in the liquid path, and refills the ejection orifice.
With the provision of the above described structure, the ejection orifices in a side shooter type liquid ejecting head in which bubbles open to the atmosphere are not plugged with recording liquid. Consequently, the appearance of the unwanted white lines during recording, for which the sudden ejection failure of some of the ejection orifices is responsible, is reliably prevented, making it possible to reliably print high quality images at a high speed.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.